The backslash-c construct was designed as a way of specifying non-printable characters,
but there were no restrictions (on ASCII platforms) on what the character following the c could be.
Now,
that character must be one of the ASCII characters.

Several changes have been made to the way die, warn, and $@ behave, in order to make them more reliable and consistent.

When an exception is thrown inside an eval, the exception is no longer at risk of being clobbered by code running during unwinding (e.g., destructors). Previously, the exception was written into $@ early in the throwing process, and would be overwritten if eval was used internally in the destructor for an object that had to be freed while exiting from the outer eval. Now the exception is written into $@ last thing before exiting the outer eval, so the code running immediately thereafter can rely on the value in $@ correctly corresponding to that eval.

Likewise, a local $@ inside an eval will no longer clobber any exception thrown in its scope. Previously, the restoration of $@ upon unwinding would overwrite any exception being thrown. Now the exception gets to the eval anyway. So local $@ is safe inside an eval, albeit of rather limited use.

Exceptions thrown from object destructors no longer modify the $@ of the surrounding context. (If the surrounding context was exception unwinding, this used to be another way to clobber the exception being thrown. Due to the above change it no longer has that significance, but there are other situations where $@ is significant.) Previously such an exception was sometimes emitted as a warning, and then either string-appended to the surrounding $@ or completely replaced the surrounding $@, depending on whether that exception and the surrounding $@ were strings or objects. Now, an exception in this situation is always emitted as a warning, leaving the surrounding $@ untouched. In addition to object destructors, this also affects any function call performed by XS code using the G_KEEPERR flag.

$@ is also no longer used as an internal temporary variable when preparing to die. Previously it was internally necessary to put any exception object (any non-string exception) into $@ first, before it could be used as an exception. (The C API still offers the old option, so an XS module might still clobber $@ in the old way.) This change together with the foregoing means that, in various places, $@ may be observed to contain its previously-assigned value, rather than having been overwritten by recent exception-related activity.

Warnings for warn can now be objects, in the same way as exceptions for die. If an object-based warning gets the default handling, of writing to standard error, it will of course still be stringified along the way. But a $SIG{__WARN__} handler will now receive an object-based warning as an object, where previously it was passed the result of stringifying the object.

A bug has been fixed when deparsing a nextstate op that has both a change of package (relative to the previous nextstate), or a change of %^H or other state, and a label. Previously the label was emitted first, leading to syntactically invalid output because a label is not permitted immediately before a package declaration, BEGIN block, or some other things. Now the label is emitted last.

The protocol for unwinding the C stack at the last stage of a die has changed how it identifies the target stack frame. This now uses a separate variable PL_restartjmpenv, where previously it relied on the blk_eval.cur_top_env pointer in the eval context frame that has nominally just been discarded. This change means that code running during various stages of Perl-level unwinding no longer needs to take care to avoid destroying the ghost frame.

The format of entries on the scope stack has been changed, resulting in a reduction of memory usage of about 10%. In particular, the memory used by the scope stack to record each active lexical variable has been halved.

Memory allocation for pointer tables has been changed. Previously Perl_ptr_table_store allocated memory from the same arena system as SV bodies and HEs, with freed memory remaining bound to those arenas until interpreter exit. Now it allocates memory from arenas private to the specific pointer table, and that memory is returned to the system when Perl_ptr_table_free is called. Additionally, allocation and release are both less CPU intensive.

A new function, Perl_magic_methcall has been added that wraps the setup needed to call a magic method like FETCH (the existing S_magic_methcall function has been renamed S_magic_methcall1).

If you find what you think is a bug, you might check the articles recently posted to the comp.lang.perl.misc newsgroup and the perl bug database at http://rt.perl.org/perlbug/ . There may also be information at http://www.perl.org/ , the Perl Home Page.

If you believe you have an unreported bug, please run the perlbug program included with your release. Be sure to trim your bug down to a tiny but sufficient test case. Your bug report, along with the output of perl -V, will be sent off to perlbug@perl.org to be analysed by the Perl porting team.

If the bug you are reporting has security implications, which make it inappropriate to send to a publicly archived mailing list, then please send it to perl5-security-report@perl.org. This points to a closed subscription unarchived mailing list, which includes all the core committers, who be able to help assess the impact of issues, figure out a resolution, and help co-ordinate the release of patches to mitigate or fix the problem across all platforms on which Perl is supported. Please only use this address for security issues in the Perl core, not for modules independently distributed on CPAN.